British tourist Hannah Witheridge was raped by two men before she was murdered, Thai police have said.

The 23-year-old, from Great Yarmouth, Norfolk, was battered to death along with 24-year-old David Miller on the island of Koh Tao on 15 September.

Autopsy results had seemingly ruled out rape, but Thai detectives have since analysed two different samples of semen from Witheridge and concluded they belong to two Asian men.

Lieutenant general Panya Mamen also said he believed a third man witnessed the attack as he confirmed investigators were now looking for the three men.

"Two of the suspects raped and killed Witheridge while another one witnessed the murder," he said.

"We're confident we have a very high chance of finding the suspects. I'm not worried because I know how much the probe has been progressing. We just have to follow forensic results."

The news came after it emerged on Friday that seven Thai football players who were celebrating on the island the day before the pair's murder provided DNA samples, reported the Bangkok Post.

Police have set up a special team to investigate rumours and tips from the public, while staff are working around the clock to analyse more than 200 DNA tests.